This invention relates to antennae which are capable of being used for signal transmission and/or reception purposes at radio and microwave frequencies.
Antennae capable of operating in the radio and/or microwave frequency range are commonly attached to, or incorporated within, electrical equipment for use in the home, commercial or laboratory environments, such as televisions, radios, cellular telephones, and wireless local area networks (WLANS). One known type of antenna is the conventional "loop-type" antenna which essentially consists of a tuned loop of wire designed to receive or radiate signals at frequencies falling within a relatively narrow usable frequency bandwidth which may, typically, be approximately 50-60 MHz wide. Such antennae have the disadvantage of having directional radiation properties the effect of which is that the antenna needs careful orientational adjustment in order to operate effectively. Moreover, the antenna is incapable of receiving or radiating signals at frequencies falling outside its relatively narrow operational bandwidth.
Another known type of antenna is the "quarter wave" antenna which generally consists of an elongate antenna element whose length is chosen to be equal to one quarter of the wavelength at the desired optimum operating frequency of the antenna and which operates against an electrical ground which is usually provided by the earthed outer conducting element of a coaxial cable, the antenna element being connected to the central signal carrying element of the cable. Although such antennae have omni-directional radiation characteristics, they also have the disadvantage of operating effectively only over a fixed, relatively narrow frequency bandwidth. The antenna element of such quarter-wave antennae may often be telescopic such that the length of the element may be varied by a user. Such antennae still require careful adjustment by the user in order to operate at different frequencies which can be time consuming and frustrating for the user. Such variable length antennae are, moreover, unsuitable for use in, for example, local area networks and cellular radio communications applications where constant antenna adjustment is not a viable option for maintaining effective operation.